1. Field Of The Invention
The present invention relates to mixing of polymer gel agents and water to form a well treatment fluid, such as a fracturing ("frac") gel or other similar gel, and more particularly, to a method and apparatus for continuously mixing such gels on a real time basis to achieve rapid hydration without the necessity of an oil-based fluid or the suspension agents normally associated therewith.
2. Description Of The Prior Art
Many treatments and procedures are carried out in industry utilizing high viscosity fluids to accomplish a number of purposes. For example, in the oil industry, high viscosity aqueous well treating fluids or gels are utilized in treatments to increase the recovery of hydrocarbons from subterranean formations, such as by creating fractures in the formation, acidizing the formations, etc. High viscosity aqueous fluids are also commonly utilized in well completion procedures. For example, during the completion of a well, a high viscosity aqueous completion fluid having a high density is introduced into the well to maintain hydrostatic pressure on the formation which is higher than the pressure exerted by the fluids contained in the formation, thereby preventing the formation fluids from flowing into the well bore.
High viscosity treating fluids, such as fracturing or acidizing gels, are normally made using dry polymer additives or agents which are mixed with water or other aqueous fluids at the job site. Such mixing procedures have some inherent problems, particularly on remote sites or when large volumes are required. For example, special equipment for mixing the dry additives with water is required, and problems such as chemical dusting, uneven mixing, lumping of gels while mixing and extended preparation and mixing time are involved. In addition, the mixing and physical handling of large quantities of dry chemicals require a great deal of manpower, and when continuous mixing is required, the accurate and efficient handling of dry chemicals is extremely difficult.
The lumping of gels occurs because the initial contact of the polymer with water results in a very rapid hydration of the outer layer of particles which creates a sticky, rubbery exterior layer that prevents the interior particles from contacting water. The net effect is formation of what are referred to as "gel balls" or "fish eyes". These hamper efficiency by lowering the viscosity achieved per pound of gelling agent and also by creating insoluble particles that can restrict flow both into the well formation and back out of it. Thus, simply mixing the untreated polymer directly with water is not a very successful method of preparing a smooth homogeneous gel free from lumps. A method directed to solving this problem is to control particle size and provide surface treatment modifications to the polymer. It is desired to delay hydration long enough for the individual polymer particles to disperse and become surrounded by water so that no dry particles are trapped inside a gelled coating to form a gel ball. This can be achieved by coating the polymer with materials such as borate salts, glyoxal, non-lumping HEC, sulfosuccinate, metallic soaps, surfactants, or other materials of opposite surface charge to the polymer.
One way to improve the efficiency of polymer addition to water and derive the maximum yield from the polymer is to prepare a stabilized polymer slurry (SPS), also referred to as a liquid gel concentrate (LGC). The liquid gel concentrate is premixed and then later added to the water. In U.S. Pat. No. 4,336,145 to Briscoe, assigned to the assignee of the present invention, a liquid gel concentrate is disclosed comprising water, the polymer or polymers, and an inhibitor having the property of reversibly reacting with the hydratable polymer in a manner wherein the rate of hydration of the polymer is retarded. Upon a change in the Ph condition of the concentrate such as by dilution and/or the addition of a buffering agent (Ph changing chemical) to the concentrate, upon increasing the temperature of the concentrate, or upon a change of other selected condition of the concentrate, the inhibition reaction is reversed, and the polymer or polymers hydrate to yield the desired viscosified fluid. This reversal of the inhibition of the hydration of the gelling agent in the concentrate may be carried out directly in the concentrate or later when the concentrate is combined with additional water.
The aqueous-based liquid gel concentrate of Briscoe has worked well at eliminating gel balls and is still in routine use in the industry. However, aqueous concentrates can suspend only a limited quantity of polymer due to the physical swelling and viscosification that occurs in a water-based medium. Typically about 0.8 pounds of polymer can be suspended per gallon of the concentrate.
By using a hydrocarbon carrier fluid, rather than water, higher quantities of solids can be suspended. For example, up to about five pounds per gallon of polymer may be suspended in a diesel fuel carrier. Such a liquid gel concentrate is disclosed in U.S. Pat. No. 4,722,646 to Harms and Norman, assigned to the assignee of the present invention. Such hydrocarbon-based liquid gel concentrates work well but require a suspension agent such as an organophylic clay or certain polyacrylate agents. The hydrocarbon-based liquid gel concentrate is later mixed with water in a manner similar to that for aqueous-based liquid gel concentrates to yield a viscosified fluid, but hydrocarbon-based concentrates have the advantage of holding more polymer.
An additional problem with prior methods using liquid gel concentrates occurs in offshore situations. The service vessels utilized to supply the offshore locations have a limited storage capacity and must therefore often return to port for more concentrate before they are able to do additional jobs, even when the liquid gel concentrate is hydrocarbon-based. Therefore, it would be desirable to be able to continuously mix a well treatment gel during the actual treatment of the subterranean formation from dry ingredients. For example, such an on-line system could satisfy the fluid flow requirements for large hydraulic fracturing jobs during the actual fracturing of the subterranean formation by continuously mixing the fracturing gel.
One method and apparatus for continuously mixing a fracturing gel is disclosed in U.S. Pat. No. 4,828,034 to Constien et al., in which a fracturing fluid slurry concentrate is mixed through a static mixer device on a real time basis to produce a fully hydrated fracturing fluid during the actual fracturing operation. This process utilizes a hydrophobic solvent which is characterized by a hydrocarbon such as diesel as in the hydrocarbon-based liquid gel concentrates described above.
Recently, however, there have been some problems with hydrocarbon-based liquid gel concentrates because some well operators object to the presence of these fluids, such as diesel, even though the hydrocarbon represents a relatively small amount of the total fracturing gel once mixed with water. Also, there are environmental problems associated with the clean-up and disposal of well treatment gels containing hydrocarbons. These hydrocarbon-related problems would also apply to the process of Constien et al. Accordingly, there is a need for a process to produce a well treatment gel in which relatively higher amounts of polymer per unit volume can be utilized while eliminating the environmental problems and objections related to hydrocarbon-based concentrates. There is also a need for this process to produce the well treatment gel substantially continuously during the well treatment operation to overcome the storage capacity problems discussed above.
The method and apparatus of the present invention provide a solution to these problems by providing a means for substantially continuously producing a fracturing gel without the use of hydrocarbons or suspension agents, while still avoiding gel balls, by feeding the polymer into an axial flow mixer which has high mixing energy to substantially wet all of the polymer during its initial contact with water. After initial mixing, additional water may be added to the mixer to increase the volume of water-polymer slurry produced thereby.
In the present invention, it is possible to use a non-coated (non-surface-treated) gelling agent. This provides a simpler and less expensive process, and the materials themselves are also cheaper because raw gelling agents are less expensive than coated or treated materials.